This invention relates to the vapor deposition of ceramic materials, and, more particularly, to improving the quality of vapor-deposited ceramic coatings by improving the feedstock used in the vapor deposition process.
Ceramic coatings are used to protect and insulate substrates in a number of applications. Ceramics usually exhibit low thermal and electrical conductivities, as well as high melting points. Most are stable at elevated temperatures. Many ceramics are resistant to oxidation, corrosion, or other forms of degradation to which metals are usually more prone. Unfortunately, most ceramics have low ductility that limit their usefulness as structural materials, although much research is devoted toward remedies for this shortcoming. In the meantime, however, the ceramics find an important use as protective coatings for underlying metallic substrates.
For example, in some advanced aircraft engines the superalloy turbine blades and vanes are coated with a thin ceramic thermal barrier coating. The thermal barrier coating acts as an insulator so that the turbine components can operate at gas-path temperatures higher than would otherwise be possible. It also protects the substrate against mechanical damage such as erosion, nicks, and dings from the impact of foreign objects ingested by the engine, referred to as foreign object damage (FOD). The thermal barrier coating may be used in conjunction with other layers deposited upon the metallic substrate, such as a bond coat, that inhibit oxidation and corrosion. The resulting thermal barrier coating system deposited upon the metallic turbine component enhances its performance by permitting it to operate at higher gas temperatures than otherwise possible and also prolongs its operating life.
The deposition of a thin ceramic coating onto a metallic substrate is itself a technically challenging operation. Since the coating is thin, it must be highly uniform. Even small nonuniformities can bridge through the entire thickness of the coating or cause such defects in the coating that it becomes vulnerable to premature failure.
One of the most popular techniques used to deposit a thin ceramic coating is physical vapor deposition (PVD). In one PVD process, the top end of an ingot of the ceramic material to be deposited is placed in a chamber which is then evacuated. The chamber can accommodate small ingots having a weight of about five (5) pounds. The typical size of an ingot is about 8" in length and 2.5" in diameter. The ingot then is heated by an intense beam of electrons or other heating source such as a laser. The heated end of the ingot is melted to form a molten pool. The heating of the surface of the molten pool is so intense that molecules of the ceramic evaporate from the melted surface. The object to be coated is positioned above the molten pool, and the evaporated molecules deposit upon the object to gradually build up a coating. A translation mechanism moves the ingot slowly upwardly to replace the evaporated material of the molten pool with additional ceramic, permitting the deposition to be performed in a continuous manner. Equivalently, pellets of the material to be evaporated having a size of about 0.125" to about 1" in diameter, instead of ingots, may be supplied and continuously fed to the vaporization apparatus.
When coatings produced by conventional electron beam physical vapor deposition (EB-PVD) apparatus are examined closely, there are often observed a large number of small chunks or pieces o,f inhomogeneous ceramic material embedded in the otherwise-homogeneous ceramic coating. These chunks of inhomogeneous ceramic material can separate from the coating during service of the coated article, leaving pinholes through the ceramic coating. The pinholes permit the hot gas to penetrate to the underlying bond coat and metallic substrate, quickly burning and/or corroding it and causing failure of the turbine component.
Efforts to reduce this problem have heretofore centered on improving the heating pattern of the molten pool. In EB-PVD, for example, an intense electron beam is repeatedly passed over the surface of the molten pool in a preselected pattern. It has been found that altering the beam intensity and heating pattern has yielded some improvement in coating uniformity. Nevertheless, even with optimization of the heating of the molten pool, inhomogeneities in the applied ceramic coating remain a problem. These inhomogeneities are caused by instabilities in the process which manifest themselves as spits, which is an ejection of a particle from the molten pool. A multitude of such ejections is called an eruption. Both events lead undesirable macroparticles being incorporated into the coating.
There is a need for an approach to understanding the origin of the inhomogeneities in the ceramic coatings and avoiding, or at least minimizing, the incidence of these features. The present invention fulfills this need, and further provides related advantages.